1. Field of the Invention
The present invention relates generally to adaptive optics and active compensated imaging systems. More particularly, the invention relates to adaptive optics and imaging systems for wavefront aberration compensation with both phase reversal and amplitude preservation.
2. Description of the Related Art
Adaptive optics and compensated imaging systems are typically used to correct wavefront aberrations in input laser beams. Typically, the laser beam is transmitted through the atmosphere from a remote source to a receiver or detector. Atmospheric turbulence can severely aberrate the laser beam. Therefore, adaptive optics and compensated imaging systems are used at the receiving site to compensate in real-time for these aberrations and perform wavefront correction or wavefront "scrubbing".
Adaptive optics systems typically employ spatial phase modulators which do not preserve the amplitude information of the incident beam; instead these systems rely primarily on phase reversal or wavefront reversal. The ability of phase conjugators to restore severely aberrated waves to their original unaberrated state after passage through the aberration-producing medium twice, and the application of phase conjugators to adaptive optics systems is well known. See for example, the discussion by C. R. Giuliano in "Applications of Optical Phase Conjugation", Physics Today, April 1981, pp. 1-8. However, the conventional approach to wavefront correction of laser beams does not compensate for amplitude variations, but only compensates for phase variations (see page 5 of the Giuliano article referred to above). This is a drawback with conventional adaptive optics systems such as the deformable mirror arrangement. Moreover, it has been theoretically shown that only if an aberrated input beam is corrected to have both wavefront reversal and amplitude preservation can it perfectly compensate for phase distortions as it propagates back through the aberrating medium. Therefore, perfect compensation is not achieved when only the phase of the aberrated beam is reversed.
It has been suggested that the presence of amplitude variations, due to atmospheric scintillation and device inhomogeneties, may seriously degrade the performance of adaptive optics systems in certain operational modes. For example, in a liquid crystal light valve (LCLV) adaptive optics system, although small amplitude variations may not degrade performance significantly, very large amplitude fluctuations may be problematic. An adaptive optics system for phase compensation is described by Cardinal Warde et al. in "High Resolution Adaptive Phase Compensation for Low-Visibility Optical Communication",Proc. IEEE, Vol. 68, pp. 539-545 (1980).
Although nonlinear optical approaches to adaptive optics applications, such as Stimulated Brillouin scattering, stimulated Raman scattering, four-wave mixing, and two-wave mixing, are able to perform aberration compensation even if the reference wave has substantial amplitude variation over the wavefront, these approaches suffer from various drawbacks. For example, the nonlinear approach generally requires the reference wavelength to meet specific requirements depending on the amplification process and the conjugation process. Also, nonlinear optical approaches are generally unsatisfactory in low-irradiance environments.
The loss of spatial amplitude information is particularly pronounced and problematic in the far-field of free-space or guided-wave structures, and in multi-mode fiber optic transmission systems.
Therefore, there is a need for an adaptive optics system which provides amplitude preservation in addition to phase aberration compensation with high spatial bandwidth. Such a system will be able to preserve or restore phase and amplitude information in an input aberrated beam even in low irradiance environments.